Abstract
1. The disease of rice-seedlings known under the name of “Bakanaebyo” is one of the most important pests of this main crop in Nippon. In all probability the disease in question exists in all parts of the Far East, where the rice plants are cultivated.
2. According to the previous investigations, the spread of the disease is clearly in close relations with the environmental conditions before and after the flowering period of the host plant, those greatly influencing the germination of conidia of its causal fungus.
3. In the present paper the results of the writers' investigations on the relation of temp rature, humidity, sunlight and the filtrates of cultures of various fungi to germination of the macroconidia of the causal fungus, Gibberella Fujikuroi, were reported.
4. Repeated experiments showed that the optimum temperature for germination of the macroconidia seems to lie at 28°-30°C. The highest limit of temperature for germination of the macroconidia was presumed to lie at a temperature slightly higher than 44°C, the lowest limit for the same being at a temperature lower than 8°C.
5. In order to decide the effect of air-humidity upon the germinabilify of macroconidia of the causal fungus, small drops of suspension were placed on clean slide glasses and dried at 28°C. Those slides were kept on supports in special dishes, in which the air-humidity had been controlled. Those dried macroconidia on slides kept in a saturated atmosphere could germinate fairly well, averaging 45.5% at 28°C.
6. The macroconidia on slides kept at 97.5% relative humidity germinated slightly at 28°C, but not at 24°C under experimental conditions. Microscopical examination demonstrated that the formation of a thin film of water surrounding the conidia is necessary for their germination.
7. The percentage of germination and also the length of germ-tubes were greater in the absence of light than in the presence of light.
8. The filtrate of cultures of the causal fungus on potato-decoction showed a tendency to accelerate slightly the germination of the same fungus, but those of Ophiobolus Miyabcanus and Piricularia Oryzae accelerating it more clearly than that of the former. The ability of acceleration seemed to diminish by prolongation of cultural period.
9. In the experimental conditions, the ability of acceleration in the filtrates was able to resist highly to heating, but the filtrates passed through a porous filter showed a tendency to check the conidial germination of the fungus.
10. The changes in pH value and the osmotic pressure of the culture-solutions caused by the growth of those fungi seemed to have no relation with the accelerating as well as the checking power of filtrates.
